1. Field of the Invention
The present invention relates to a fault detector for a vehicle safety system, and more particularly to an inexpensive and highly reliable fault detector for a detecting a fault of a trigger resistor which is installed in a vehicle air bag system and the like.
2. Description of the Prior Art
Heretofore, a trigger heater having a relatively small resistance value called a "squib" has been used in a vehicle air bag system for expanding the air bag instantaneously upon a vehicle crash. The resistance of such a trigger resistor must be constantly monitored to determine whether it is normal or abnormal since the value of the trigger resistor relates to passenger safety.
JA. Pat. Publication No. 61-57219 shown as a block diagram in FIG. 3 is one of the fault detectors for vehicles in accordance with the prior art.
In the drawing of FIG. 3, element (1) is a battery or a DC power source installed in a vehicle, element (2) is an ignition switch connected to the DC power source (1) for the starting engine, element (3) is an acceleration sensor (hereinafter called a G sensor) connected to the DC power source (1) via the ignition switch (2) and consists of a parallel circuit of a normally open switch (31) and a resistor (32), and element (4) is the squib (the resistor to be checked) connected to the G sensor at a junction A for expanding the air bag and consisting of a series circuit together with the resistor (32) in G sensor (3).
Element (5) is another G sensor connected to the squib resistor (4) at a junction C and consists of a parallel circuit of a normally open switch (51) and a resistor (52) in the same way as G sensor (3) whereby the other end is grounded.
Circuit (6) is a fault detecting circuit connected to both ends of the squib resistor (4) as well as G sensors (3) and (5) for detecting a fault of the squib resistor (4) and is consisted of a DC differential amplifier circuit (7) connected to junctions A and C and a comparator circuit (8) connected to an output terminal of said differential amplifier (7).
The differential amplifier circuit (7) is comprised of resistors (71)-(74) for determining the gain thereof and an operational amplifier (75), whereby the resistor (71) is connected between junction A and a non-inverted input terminal of the operational amplifier (75), resistor (72) is connected between the ground and the non-inverted input terminal of the operational amplifier (75), the resistor (73) is connected between junction C and an inverted input terminal of the operational amplifier (75) and the resistor (74) is connected between the output terminal and the inverted input terminal of the operational amplifier (75) respectively.
The comparator circuit (8) is comprised of a series of connected resistors (81)-(83) for providing a standard voltage by dividing the voltage of DC power source (1), an operational amplifier (84) wherein the non-inverted input terminal is connected between a junction of resistors (81) and (82) and the inverted input terminal is connected to the output terminal of the operational amplifier (75), an operational amplifier (85) wherein the inverted input terminal is connected between a junction of resistors (82) and (83) and the non-inverted input terminal is connected to the output terminal of the operational amplifier (75), and an AND gate (86) for multiplying the outputs of the operational amplifiers (84) and (85). Further, element (9) is a warning lamp connected to the output terminal or the output terminal of the AND gate (86).
The operation of the prior art fault detector shown in FIG. 3 will now be described more in detail, wherein, the G sensors (3) and (5), the squib resistor (4) and a fault detector circuit are supplied power from the DC power source (1) simultaneously upon the closure of the ignition switch (2) for starting the engine of the vehicle and a voltage is generated across the squib resistor (4). The output voltage V1 of the DC power source (1) is then divided by the resistors (32) and (52) and the squib resistor (4).
In such an arrangement, since the resistance value R4 of the squib resistor (4) small while the resistance values R3 and R5 for resistors (32) and (52) are large and the output voltage V1 is approximately 12 V, the voltage difference Vac between the junctions A and C will be several 100 mV. For an example: if EQU R3=R5=1k.OMEGA. EQU R4=2.OMEGA.
the voltage Vac across the squib resistor (4) will be: ##EQU1## whereby, if the G sensor is shorted, the voltage Vac will be given by: ##EQU2## and, if the squib resistor (4) is shorted, the voltage Vac will be given by: EQU Vac=0 V
Accordingly, in order to determine a fault based on a voltage that varies within a range of 0-10 mV, it is necessary to adjust the resistance values of resistors (71)-(74) to making the gain of the differential amplifier (75) to be 100 or so. With this adjustment, the output voltage V7 of the differential amplifier (7) will be 1.2 V in a normal condition and it will be 0 V if the squib resistor (4) is shorted.
Therefore, the resistors (81)-(83) in the comparator circuit (8) are adjusted so that the output voltages of the operational amplifiers (84) and (85) becomes "H" level when the output voltage V7 is normal (1.2 V), the output voltage of the operational amplifier (84) becomes "L" level time when either of G sensor (3) or (5) is abnormal by being shorted (2.4 V), and the output of the operational amplifier (85) becomes "L" level when the squib resistor (4) is shorted.
It is apparent from the foregoing that the output of the AND gate (86) becomes "H" level and the lamp (9) remains OFF while in normal condition, however, it becomes "L" level in the case of abnormal condition and the lamp (9) is turned ON to warn the driver of the abnormal condition.
If the vehicle crashes when the G sensors (3) and (5) and the squib resistor (4) are in their normal state, the normally open switches (31) and (51) will close, hence, the squib resistor (4) will generate heat to trigger the expansion of the air bag to protect the driver.
In the prior art fault detector, it has been necessary to set the gain of the DC differential amplifier circuit (7) to 100 or so in order to detect a voltage variation being caused by any fault. Therefore, the fault detector of the prior art has such problems as being weak for noise because of the high gain in amplification, which causes errors due to the input offset voltage of the amplifier if a DC differential amplifier is utilized and, moreover, it is necessary to install a precision amplifier which requires precise adjustments in the stages of production, hence, resulting in the loss of the reliability as well as increasing the cost.